The present invention relates to an oil free screw compressor which synchronously rotates a pair of screw rotors without being in contact, and more particularly to an oil free screw compressor preferable for being driven by a high speed motor.
A conventional oil free screw compressor is, for example, as described in Japanese Patent Unexamined Publication No. 6-346881, structured such as to increase a rotational speed of a motor by using a belt and a gear so as to rotate a screw compressor main body. Further, in Japanese Patent Unexamined Publication No. 3-151592, there is described an embodiment of connecting a speed increasing gear apparatus receiving a speed increasing gear within a casing to a rotor shaft having a screw gear formed thereon via a coupling.
In this case, in the screw compressor, in addition to an operation control such as a load, an unload and the like, a capacity control for controlling an opening and closing operation of a suction throttling valve in accordance with a consumption requirement in the demand side is performed. As an example of the capacity control, there is described in Japanese Patent Unexamined Publication No. 59-93989 a structure in which a valve plate for the suction throttling valve is mounted to a front end of an air cylinder operated by a pressure of the compressor itself and an amount of a suction air is adjusted at two stages by moving the valve plate.
Here, the compressor described in Japanese Patent Unexamined Publication No. 6-346881 mentioned above requires a lot of parts such as a bearing for rotating and supporting a speed increasing gear, a rotary shaft for mounting the speed increasing gear, a belt and a pulley for transmitting a power having an increased speed and the like in addition to a gear case for receiving the speed increasing gear, thereby causing an increase of a cost for the compressor. Further, in this compressor, an electric motor for driving the screw rotor is enlarged, and accordingly, the compressor is insufficient in view of making small-sized a whole of the compressor unit including a stand for fixing the electric motor.
Further, in the compressor described in Japanese Patent Unexamined Publication No. 3-151592, since the speed increase is not performed by the belt, a speed increasing rate in the speed increasing gear is increased, and a gear case for receiving the speed increasing gear is enlarged. Then, in order to make the compressor as series of widely used compressors, it is necessary to combine various kinds of compressor main bodies and speed increasing gear apparatuses, thereby causing an increase of a cost in view of preparing various kinds of compressors.
Still further, in the compressor described in Japanese Patent Unexamined Publication No. 59-93989, since an air for operating a suction throttling valve is supplied to an air cylinder at every time when a line pressure is changed, a three-way electromagnetic valve is connected to an air cylinder and a supply hole for the operating air in the air cylinder is switched by the three-way electromagnetic valve. As mentioned above, since it is necessary to provide with the three-way electromagnetic valve, a structure of a flow rate control system becomes complex as well as the compressor becomes expensive. Further, in order to cancel an unloading at a time of start, a plurality of three-way electromagnetic valves are required, so that a structure of a capacity control apparatus becomes complex. In any one of the compressors mentioned above, a certain degree of consideration is given to making the compressor compact, however, a more compact structure is desired.